INFRA-RED DRYING OF ONION SLICES.

Abstract

A study was carried out to test and evaluate the use of infra-red radiation as a heat energy source for drying onion slices( Allium cepa L.) using a laboratory scale infra-red dryer. The studied parameters included four different levels of infra-red radiation intensity (0.861, 0.973, 1.039 and 1.161 kW/m²), three different levels of inlet air temperature (40, 50 and 60 oC ) and three different thicknesses of onion slices ( 2, 4 and 6 mm). All the experimental runs were conducted at air velocity of 1 m/s. Meanwhile, two different drying models, simple exponential model (Lewis's 1921) and the modified exponential model (Henderson and Pabis's 1961) were examined for describing the drying behavior and predicting the changes in moisture content of onion slices during the drying process. Quality evaluation tests were also conducted for the dried onion slices. The measured parameters included, Thiosulphinates, anthocyanin and rehydration ratio. The results showed that: 1- The drying rate of onion slices increased with the increase of radiation intensity and drying air temperature while, it was decreased with the increase of slices thickness. 2- Both studied models could describe the drying behavior of onion slices satisfactory. However, the simple exponential model (Lewis's 1921) was selected . 3- The onion slices dried at radiation intensity of 0.861 kW/m2,air temperature of 50 oC and slices thickness of 4 mm recorded the highest quality of the dried samples in terms of thiosulphinates and anthocyanin contents and the best rehydration ratio .